1. Field of the Invention
The present invention relates to an optical semiconductor device and a process of producing same.
To provide a very high speed optical communication system, an advanced optical semiconductor device has a diffraction grating formed therein for transmitting a light having a specific wavelength. Typical applications of such diffraction grating include a distributed feedback laser (DFB-LD).
2. Description of the Related Art
In the latest trend of producing these diffraction gratings, a dry etching process is often used to control the grating or corrugation height more precisely than the old wet etching process. Component crystals of an optical semiconductor device are also often formed by metalorganic chemical vapor deposition (MOCVD) to control the crystal composition and the crystal layer thickness more precisely than the old liquid phase crystal growth process.
To form a diffraction grating on an InP wafer or substrate, the surface of the InP crystal substrate is processed by dry etching such as reactive ion etching (RIE) using an ethane gas plasma to form a periodic corrugation having a height corresponding to that of the desired diffraction grating.
In the conventional process of producing an optical semiconductor device, a diffraction grating is formed with a corrugation height of about 40 nm, i e., the height applied when component crystal layers of the device are formed by a liquid phase growth process, and then component crystal layers are actually formed on the diffraction grating by MOCVD.
Generally, component crystal layers of an optical semiconductor device are formed by a three-stage crystal growth process: the first stage of growing crystal layers including an active layer to form a laminate structure on a diffraction grating or a periodic corrugation, followed by etching the laminate structure to form a mesa structure; the second stage of growing crystal layers filling both sides of the mesa structure; and the third stage of growing thereon crystal layers including an uppermost contact layer. Upper and lower electrodes are then formed to complete an optical semiconductor device.
Regarding the production of a distributed feedback laser (DFB-LD), FIG. 1 exemplifies crystal layers formed on an n-InP crystal substrate in the above-mentioned first growth stage. After forming a diffraction grating 2 in the form of a periodic corrugation on an n-InP substrate 1 by dry-etching the substrate surface, an MOCVD process is carried out to form, on the diffraction grating 2, an n-InGaAsP guide layer 3 with a composition providing a luminescence wavelength of 1.1 .mu.m, an InGaAsP active layer 4 with a composition providing a luminescence wavelength of 1.55 .mu.m, and a p-InP clad layer 5 in that order.
As shown in FIG. 2, a distributed feedback laser is produced by etching the laminate structure (from the substrate 1 through to the clad layer 5), which has been formed in the first growth stage, to form a mesa structure "M" extending perpendicular to the direction of the periodic corrugation, then forming a p-InP layer 8 and an n-InP layer 9 to fill both sides of the mesa "M", and forming thereon a p-InP layer 10, an InGaAsP contact layer 11 and upper and lower electrodes 12 and 13. Typically, the mesa "M" has a width of about 1.3 .mu.m and the cavity length is 900 .mu.m.
The inventors, however, found that a distributed feedback laser produced by the above-mentioned conventional process exhibits a poor performance including the lasing threshold current density and the luminescence efficiency in comparison with that of a device produced by a process in which wet etching is used to form a diffraction grating and a liquid phase crystal growth process is used to form component crystal layers of the device.